Modelling of pH and inorganic carbon speciation in estuaries using the composition of the river and seawater end members

doi:10.1016/j.envsoft.2010.06.014

Environmental Modelling & Software

Volume 25, Issue 12, December 2010, Pages 1658-1663

https://doi.org/10.1016/j.envsoft.2010.06.014 Get rights and content

Abstract

An equilibrium model based on the CO₂ system was developed to model pH throughout the estuarine salinity range using the composition of the river and seawater end members. In order to validate the model, a spectrophotometric method was used to measure pH in an estuary and laboratory mixing experiments. The model successfully represented the measured pH values (±0.1 pH units) for three different river water end member compositions. The pH decreased at low salinities (S ≈ 0–2), increased at intermediate salinities (S ≈ 2–15), and then stabilised for higher salinity values. Changes in the inorganic carbon speciation in the low salinity region of estuaries result in assimilation of river borne CO_2(aq).

Introduction

Natural waters contain a number of weak acids and bases that control the hydrogen ion activity as measured by the pH parameter. By far the most abundant of these species are carbonate and bicarbonate ions which originate mainly from the dissolution of carbonate rocks, atmospheric CO₂ exchange, and the respiration of aquatic organisms (Cai et al., 2008). Seawater on average has more than twice the inorganic carbonate concentration of river water (Morel, 1983). Estuaries act as an important mixing zone between freshwater systems and the open ocean and along with coastal marsh areas, are considered to make a substantial contribution to the global carbon budget (Cai et al., 1999). The total inorganic carbon and alkalinity may mix conservatively with salinity in an estuary but the pH and partial pressure of CO₂ (pCO₂) do not (Morel, 1983, Cai and Wang, 1998). Better characterisation of the CO₂ system in estuaries is important given that increasing anthropogenic CO₂ emissions are predicted to result in large changes in the pH of natural waters (Millero, 2007).

The increased interest in understanding the CO₂ system in the oceans has led to the thermodynamic equilibrium constants being characterised over the salinity range from 0 to 45 (Dickson and Millero, 1987, Roy et al., 1993, Millero, 1995). This has led to successful validation of equilibrium models of the CO₂ system in seawater (Millero et al., 2002, Ohline et al., 2007) but not to our knowledge in estuaries. The difficulty of measuring pH accurately in estuaries using conventional glass electrodes (Butler et al., 1985, Whitfield et al., 1985, Millero, 1986), has also complicated the characterisation of the CO₂ system. Indicator-based spectrophotometric measurement techniques have been used for the precise and accurate determination of pH in seawater for decades (Robert-Baldo et al., 1985) and recently these techniques have been extended to estuaries (Mosley et al., 2004). This has enabled the accurate determination of pH and the validation of models for study of the CO₂ system and other pH-dependent biogeochemical processes in estuaries (e.g. trace metal sorption, Wang and Liu, 2003).

In the present study, the pH and inorganic carbon speciation were modelled in estuaries using an equilibrium model of the CO₂ system based solely on the river and seawater end member composition. To validate the model, a spectrophotometric method was used to accurately determine pH in an estuary and laboratory mixing experiments. This modelling should be of wider interest to other researchers as they provide to our knowledge the first attempt to validate pH and related CO₂ system models throughout the estuarine salinity range.

Section snippets

Estuarine pH modelling

The carbonate system in natural waters can be fully characterised by measurement of two of the four parameters, total alkalinity, total carbonate, pH and pCO₂, and any other factor can be calculated using known thermodynamic relations (Millero, 1995, Stumm and Morgan, 1996). In theory, if any two of these parameters are measured for the end members of an estuary, the characteristics of the CO₂ system throughout the salinity range in the estuary can be predicted from knowledge of the various

Results and discussion

The composition of the river and seawater end members used in the modelling calculations is given in Table 1. The modelled pH profile with salinity in the Taieri Estuary and laboratory mixing experiments is shown in Fig. 1, Fig. 2 together with the measured pH values.

The field and laboratory mixing pH measurements showed similar trends with the pH decreasing slightly at low salinities (S < 2), increasing rapidly at intermediate salinities (S ≈ 2–15), and then stabilising at higher salinities (S

Conclusion

pH and inorganic carbon cycling in estuaries can be more accurately studied through the methods outlined in this paper. The pH profiles observed in the field study and laboratory experiments were successfully modelled in XLCO2 using the measured composition of the river and seawater end members as input. The speciation of the inorganic carbon components was also calculated and provided an insight into a previously unknown process that removes river borne CO_2(aq) under low salinity conditions.

Acknowledgements

Thanks to Melissa Tapp for assistance with the spectrophotometric pH measurements and data analysis, and Bill Dickson for assistance with field work.

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Modelling of pH and inorganic carbon speciation in estuaries using the composition of the river and seawater end members

Abstract

Introduction

Section snippets

Estuarine pH modelling

Results and discussion

Conclusion

Acknowledgements

Marine Chemistry

Geochimica et Cosmochimica Acta

Geochimica et Cosmochimica Acta

Continental Shelf Research

Estuarine, Coastal and Shelf Science

Deep-Sea Research I

Geochimica et Cosmochimica Acta

Deep-Sea Research

Continental Shelf Research

Geochimica et Cosmochimica Acta

Deep-Sea Research

Geochimica et Cosmochimica Acta

Deep-Sea Research I

Marine Chemistry

Marine Chemistry

Marine Chemistry

Marine Chemistry

Analytica Chimica Acta

Marine Chemistry

Marine Chemistry

Chemical Geology

Marine Chemistry